We investigated the role of two anti-human colon tumor antigen monoclonal antibodies (MAb), chimeric B72.3 (mouse/human)(cB72.3) and native murine D612, in directing and augmenting antibody dependent cellular cytotoxicity (ADCC) activity. cB72.3 antibodies have a human IgG1 (gamma1) or IgG4 (gamma4) constant region. The native murine B72.3 (nB72.3) MAb reacts with antigen expressed by many human carcinomas including colon, breast and ovary but not by most normal adult tissues. D612 (IgG2a, K) reacts with a 48Kd glycoprotein found on the membrane of malignant and benign human gastrointestinal epithelium but not other adult normal or malignant tissues. ADCC activity of cB72.3 was compared to nB72.3. The xenografted OVCAR-3 human ovarian carcinoma ascites were used as a target in the cytotoxic assay. Lytic activity of the cB72.3 (gamma1) MAb with human peripheral blood mononuclear cells (PMNC) was 1.5 to 50 fold greater than that of the nB72.3 and cB72.3 (gamma4). Exposure of PMNC to interleukin-2 (IL-2) for 24h to generate LAK cells augmented ADCC mediated by the cB72.3 (gamma1) MAb 2 to 22 fold. Depletion of FcR(gamma)III positive cells in the PMNC markedly reduced the D612 ADCC. We also studied the effect of human rIL-6 (hrIL-6) on ADCC activity mediated by human PMNC. The ability of hrIL-6 to augment ADCC was measured using D612 MAb and colorectal carcinoma targets LS-174T, WiDr, and HT-29. A significant increase in ADCC activity was observed after PMNC were incubated in 100-400 U/ml of hrIL-6. Human rIL-6 did not augment non-specific (non MAb mediated) cytotoxicity. Enhancement of ADCC activity was blocked by the addition of an antibody against hrIL-6 but not by an antibody to the IL-2 receptor, suggesting the hrIL-6 augmentation of ADCC activity may not be mediated through IL-2. This shows that cB72.3 (gamma1) MAb and murine D612 MAb have appreciable ADCC mediating properties and hrIL-6 can augment ADCC activity of human PMNC using MAbs to human tumor antigen and human tumor cells as targets. Results obtained from these studies suggest the cB72.3 and D612 should be considered as candidates for immunotherapy of colon cancer. Furthermore, the data also suggest a potential role of IL-6 in combination with anti-cancer antibodies for cancer immunotherapy.